In general, lithographic printing plates are comprised of oleophilic image portions receiving inks during the printing step and hydrophilic non-image portions receiving dampening water. As such lithographic printing plates, PS plates comprising an oleophilic light-sensitive resin layer provided on a hydrophilic support have hitherto been widely used. In the conventional process works of PS plates, after the exposure, an operation of dissolution and removal of the non-image portions with a highly alkaline development processing liquid is needed. In the conventional techniques, one of problems that should be improved was to make such an additional wet processing simple or unnecessary. Especially, in recent years, disposal of wastes to be discharged following the wet processing is being a great matter of concern over the whole of the industrial field from consideration to the global environment, and therefore, a demand of improvement in this issue becomes strong more and more.
On the other hand, in recent years, as another trend of this field, digitization techniques of electronic processing, accumulation and outputting of image information using a computer become widespread, and various new image-outputting modes responsible to such digitization techniques have been put into practical use. Following this, computer-to-plate techniques of carrying the digitized image information on a highly convergent radiation such as laser, scanning exposing a printing precursor with this light, and directly manufacturing a printing plate without using a lith film are watched.
Especially, in recent years, high-output solid lasers such as semi-conductor laser and YAG laser have become available cheaply. Accordingly, the printing plate preparation work with high power-density exposure using a high-output laser has become promising. According to this preparation work, the exposed region is convergently irradiated with a large dose of light energy during an instantaneous exposure time, to efficiently convert the light energy into a heat energy, and the heat causes chemical change, phase change, and heat changes such as change of shape or structure, thereby utilizing such changes for imagewise recording. That is, while the image information is input by the light energy such as laser, the imagewise recording is achieved by the reaction by the heat energy.
Usually, the recording mode utilizing heat generation by high powder-density exposure is called heat mode recording, and conversion of the light energy to the heat energy is called light-heat conversion.
Of these heat mode-recording lithographic printing plate precursors, thermosensitive lithographic printing plate precursors comprising, as an image forming thermosensitive layer, a hydrophilic layer having hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder polymer are promising for simple development processing. A method of using such a thermosensitive lithographic printing plate precursor utilizes a phenomenon where when a heat is applied to the thermosensitive layer, the hydrophobic thermoplastic polymer particles are fused to each other, thereby converting the surface of the hydrophilic thermosensitive layer into an oleophilic image portion.
For example, Japanese Patent No. 2,938,397, JP-A-9-127683 and WO99-10186 disclose lithographic printing plate precursors comprising a hydrophilic support having provided thereon a thermosensitive layer having fine particles of a thermoplastic hydrophobic polymer dispersed in a hydrophilic binder polymer. These patent documents describe that in such lithographic printing plate precursors, the fine particles of the thermoplastic hydrophobic polymers are united with each other by heat upon exposure with infrared laser to form an image, which is then developed in a printing machine while supplying dampening water and/or an ink (so-called “development-on-printing machine”).
However, it is difficult to thoroughly remove the thermosensitive layer of non-image portions containing such thermoplastic hydrophobic fine particles by the development-on-printing machine by dampening water or an oily ink, so that there was involved a problem that the thermosensitive layer components remain in the non-image portions to cause staining in printing.